In recent years the use of natural energy as renewable source of power has received great attention. One example is power generation with photovoltaic cells (solar cells), utilizing solar radiation. To increase the amount of power generation, a collector including reflective mirrors and lenses in combination to collect light on solar cells has been proposed (refer to Patent Document 1). For example, Patent Document 2 discloses a collector in which a large number of light-collecting Fresnel lenses are arranged on the same plane and each solar cell is arranged at the focus position of each Fresnel lens as a unit. The Fresnel lenses for such a collector are generally formed by casting, injection molding, press molding, or transcription molding as photopolymerization (2P) using a UV curable resin, using a metal mold cut from a cut metal plate with a lathe.
To arrange the solar cells at the focus positions of the Fresnel lenses, the lenses can be individually mounted in frames. For the sake of facilitating manufacturing, it is proposed that a large number of light-collecting Fresnel lenses (unit lenses) in FIG. 1 be arranged on the same plane to form a sheet (hereinafter, multiple Fresnel lens sheet) and fix it in a frame.
The Fresnel lenses need to be translucent and made from transparent resin or inorganic glass. The transparent resin is preferably used because of light weight and ease of manufacturing.
FIG. 2 schematically shows an example of a photovoltaic unit as seen from the cross section of a Fresnel lens sheet. It includes a base plate 2 (hereinafter may be referred to as solar cell plate) on which solar cells 3 are arranged and a multiple Fresnel lens sheet 1 on a light incidence side of the solar cells 3. The sheet is supported in a frame 4 so that each of the solar cells is disposed at the focus position of each Fresnel lens. The Fresnel lens sheet 1 and frame 4 may be damaged by a difference in elasticity due to a temperature or humidity change if the sheet and frame are too firmly fixed. In view of this, it is preferable that the sheet is movably held in a planar direction. Further, the solar cells are often placed on a metal plate for the purpose of heat radiation.
Meanwhile, it is necessary to sufficiently reduce a gap between the sheet and the frame in order to prevent rain drops or dust from entering inside which would otherwise cause failures. FIG. 8 shows the structure which can achieve both of the above, for example. In this structure a seal element 8 is provided at one end of the sheet 1 and the sheet 1 and a metal frame 7 are fixed with a screw 6. With such a structure the sheet can be fixed along the thickness with a sufficiently reduced gap and at the same time it is movable in a planar direction.
It is essential to accurately set the focus positions of the Fresnel lenses relative to the corresponding solar cells in view of power generation efficiency. However, the multiple Fresnel lens sheet and frame are in general made from different materials from those of the base plate on which the solar cells are disposed so that with a change in temperature or humidity, they differently behave in terms of elasticity. This may cause a displacement of the focus position of each unit lens from the solar cell, resulting in a decrease in the power generation efficiency. This is particularly significant for a resin Fresnel lens sheet.
With a change in temperature or humidity, the sheet in FIG. 8 expands and shrinks radially around a single point most firmly fixed on the frame. Therefore, a problem arises that the focus position of the unit lens in the portion furthest from the single point is greatly shifted from the corresponding solar cell.
Moreover, it is difficult to estimate which portion of the sheet will be the center point of expansion and design the sheet with the displacement taken into consideration. With use of a rectangular multiple Fresnel lens sheet, if the center point falls in the vicinity of any of the four corners, the focus position in the opposite corner will be most displaced from the solar cell.